Rotate the crankshaft until the piston is halfway down the cylinder, then install the piston stop. Now gently rotate the crankshaft until it hits the stop. Record the number of degrees indicated on the degree wheel. Then rotate the crankshaft in the opposite direction until the piston hits the stop again, and record that number. Top dead center (TDC) is exactly halfway between these two marks, so count the number of degrees between the two marks and divide by two.